Figure 1. A portion of  Dr. Snow's 1854 London street map

Figure 2. Green dots represent deaths by cholera, red the location of water wells

Figure 3. Thiessan polygons around the pumps.

Figure 4. Orange circles are the midpoints between adjacent pumps (red). These midpoints are then used to construct the Thiessan polygons

Figure 5. A single pump is responsible for the epidemic.